HDDs (Hard Disk Drives) are widely used as comparatively large-capacity storage units. In particular, HDDs referred to as near line disks are slow in access speed, but they are known to be low in price per data capacity. In recent years, on the other hand, SSDs (Solid State Drives) have spread as another type of storage unit. An SSD is higher in price per data capacity than an HDD, but the random access performance of an SSD in particular is higher than that of an HDD.
Various uses for an SSD having such high performance are discussed. For example, there is a system which uses an SSD as a cache shared by a plurality of HDDs. Furthermore, there is a system in which data is selectively written to an HDD or an SSD according to an access characteristic, that is to say, according to whether access is sequential access or random access.
Japanese Laid-open Patent Publication No. 2010-191672
Japanese Laid-open Patent Publication No. 2008-015623
Japanese National Publication of International Patent Application No. 2011-515727
By the way, with many storage systems a RAID (Redundant Arrays of Inexpensive Disks) technique is used for exercising recording control. For example, data is stored in two or more storage units for realizing redundancy. In particular, a storage system controlled by using RAID-4, RAID-5, RAID-6, or the like as a RAID level includes many storage units. As a result, large storage capacity is needed.
With a storage system using the RAID technique, on the other hand, either of an HDD and an SSD can be used as a storage unit. Access speed can be increased by using an SSD. However, this needs a large-capacity storage area by the use of an SSD, and so increases the costs of the storage system.